1. Field of the Invention
The present invention relates to an image forming apparatus, and more particularly to an image forming apparatus incorporating a fixing device that can prevent an excessive increase in a temperature.
2. Discussion of the Background
In an image forming apparatus, such as a copying machine, a facsimile machine, a printer, and other similar devices, an unfixed image that has been transferred onto a recording medium such as a transfer sheet is fixed by a fixing device and the recording medium is discharged as a hard copy. The fixing device includes a pair of rollers provided such that the rollers oppose each other. One roller functions as a heating roller. The other roller functions as a pressure roller to press a recording medium during an image fixing operation. The recording medium having an unfixed image thereon is conveyed to a nip region formed between the heating roller and the pressure roller where the unfixed image is fused and fixed onto the recording medium with heat of the heating roller. A fixing device referred to as a SURF (i.e., surface rapid fusing) type is commonly known. In the SURF type fixing device, a fixing operation is performed through a nip region formed by press-contacting a pressure roller with a heating member having a heating source via a film-shaped endless belt.
In addition, a belt-type fixing device is commonly known. In the belt-type fixing device, a heating member having a heating source and a contacting member contacting an endless belt are provided in a loop of the belt. A fixing operation is performed through a nip region formed by press-contacting a pressure roller with the contacting member via the endless belt.
An example of the belt-type fixing device includes a belt, which is spanned around a plurality of rollers. One of the plurality of rollers (e.g., a fixing roller) is positioned to oppose a pressure roller. Another roller (i.e., a heating roller) of the plurality of rollers, which drives the belt together with the fixing roller includes a heating source inside the roller. The heating source heats the belt while the roller contacts an inner surface of the belt. The pressure roller includes a heating source inside the roller to heat an outer surface of the belt. A volume and a thermal capacity of a belt is smaller than a volume and a thermal capacity of a roller. Thus, a temperature of the belt increases in a shorter period of time compared to that of the roller. An advantage of the belt-type fixing device includes a shorter warm-up time as compared to the fixing device employing the heating roller and pressure roller. In addition, because a heating source is provided inside the pressure roller, the belt is heated from both inner and outer surfaces thereof, resulting in a shorter warm-up time. In the belt-type fixing device, if each of the pair of rollers is formed of aluminum that has high thermal conductivity, the belt is formed of two layers, namely, a releasing layer that includes silicone rubber or fluorine resin layered on a substrate layer including a stainless steel.
The present invention relates to the belt-type fixing device. In the SURF type fixing device, the heating source is provided and controlled in the nip region. Thus, a temperature of the nip region is precisely controlled. Hence, a material having a low thermal capacity is selected as the endless film or heating member to increase a thermal responsivity, thereby having a minimum effect on the temperature of the nip region. However, an elastic member having a large thermal capacity is not provided in the nip region. Thus, if an elastic layer is provided on the pressure roller, a pressing operation, in which an unfixed image is sandwiched between two members having an elastic member, is barely performed. Accordingly, a nip region having a sufficient length is not formed, thereby resulting in a low level of a fixing performance. The nip region having the sufficient length is formed if the pressure roller having the elastic layer is in strong press-contact with an opposing member (i.e., heating member), due to a deformation of the elastic layer. However, the opposing member needs to have high strength. If greater rigidity is provided to the opposing member, a thermal capacity of the opposing member becomes large. In the roller-type fixing device, an elastic layer is provided to the fixing roller. However, a thermal capacity of the heating roller is increased due to the elastic layer, resulting in a long period of warm-up time.
In the belt-type fixing device, an elastic layer is provided to the contacting member because the heating member having a heating source is provided at a position other than the nip region. A temperature of the contacting member need not to be increased to a fixing temperature, but the belt alone is heated to a predetermined temperature. Thus, a long period of time is not required for a warm-up operation. An elastic layer may be provided on the belt having a higher thermal storage capacity. It is preferable that a thickness of the layer is in a range of approximately 50 μm to approximately 300 μm because if the thickness is large, a long warm-up time is required. In this case, the belt also functions as an elastic member in the nip region.
In the belt-type fixing device, an excessive heating phenomenon occurs due to a low thermal storage capacity of the belt and a heating position of the belt. A commonly known excessive heating phenomenon in the roller-type fixing device is described below. For example, approximately 90 seconds (i.e., approximately 0.6° C./sec) are generally required in the roller-type fixing device when a temperature of a surface of the roller is increased from 170° C. to 230° C. (i.e., 50° C. difference). The reason why a long period of time is required is due to a large thermal capacity of the roller. In the roller-type fixing device, the excessive heating phenomenon occurs if responsivity of a temperature detecting sensor is slow. This happens because, for example, energization of a heater is not stopped until the surface temperature reaches to approximately 230° C. even if the temperature control is arranged such that the energization is stopped when the surface temperature reaches to 170° C. In this case, if a temperature detection element having a fast responsivity is employed, the above-described problem is solved to a certain extent. The above-described excessive heating phenomenon in the belt-type fixing device occurs even when a temperature detection element having a fast responsivity is employed.
In a recent temperature detecting element, responsivity is improved. Thus, in a fixing device employing a heating member having a low thermal capacity that is heated in a short period of time (for example, in the belt-type fixing device in which a speed of a surface temperature rise is approximately 2.5° C./sec.), a difference between the actual surface temperature and a controlled surface temperature of the belt is made small. The surface temperature of the belt is increased from 170° C. to approximately 230° C. within approximately 20 seconds compared to approximately 90 seconds required in the roller-type fixing device. If the temperature detection element having a fast responsivity is used, a temperature control is arranged such that energization of heater is stopped when the actual surface temperature reaches to approximately 180° C., for example, depending on a temperature from which the surface of the belt is increased.
However, even if the temperature detection element having a fast responsivity is employed in the belt-type fixing device, the below-described excessive heating phenomenon occurs because the belt is locally heated at a position which is different from a position where heat of the belt is greatly absorbed. Namely, the surface temperature of the belt differs by about 10° C. to 20° C. between a portion of the belt that just passed through a heat absorbing region (i.e., nip region) and a portion of the belt that is about at an end of a heating position because of a low thermal storage capacity of the belt. When fixing operation of the last recording medium is completed and a portion of the belt associated with the last fixing operation is moved to a heating position, the portion of the belt is heated by a heating member.
Heat of the heating member is thus absorbed and a temperature of the heating member decreases which is detected by a temperature detection element. Thus, a heater of the heating member is turned on. However, even though the temperature detection element having a fast responsivity is employed, the heating member heats a portion of the belt that is behind the portion of the belt associated with the last fixing operation. Because heat of this portion of the belt is not absorbed by a recording medium, a temperature of this portion is further increased even though the temperature of this portion is higher than the portion of the belt associated with the last fixing operation by about 10° C. to 20° C. Then, the surface temperature of the belt differs by about 15° C. to 30° C. between the highest temperature portion and the lowest temperature portion. Thus, an excessive heating phenomenon occurs. An excessive amount of heat is applied to a recording medium which causes a hot offset phenomenon or produces an adverse effect on glossiness of an image. In addition, an excessive temperature increase inhibiting device, such as a thermal fuse and temperature thermostat is damaged due to an increase of a temperature in a fixing unit. If the belt keeps on rotating under this condition, the surface temperature of the belt is gradually made uniform such that the surface temperature is maintained at a predetermined temperature. However, if the rotation of the belt is stopped for an energy saving purpose, a longer period of time is required before the surface temperature of the belt is made uniform. If the portion of the belt associated with the last fixing operation stops at the nip region, heat of this portion of the belt is absorbed in the nip region (i.e., by a pressure roller). Then, a temperature of this portion of the belt further decreases. Thus, a difference in a temperature between the portion associated with the last fixing operation and the portion of the belt behind the portion associated with the last fixing operation further increases. If the temperature detection element is provided to detect a surface temperature of a heating member including a heating source instead of detecting a surface temperature of the belt at a heating position, a decrease of temperature of the heating member is detected instead of a decrease of temperature of the belt, resulting in a slow response of the temperature detection element, and a delay in controlling a heater.